low-power audio amp basics

Yes

It's to provide amplifier headroom. With a single supply, the speaker is capacitively coupled to the amplifier output.

...Jim Thompson

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| James E.Thompson                                 |    mens     | 
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I love to cook with wine.     Sometimes I even put it in the food.

Normally with single ended supplies, you'd couple the speaker with a DC blocking capacitor - big electrolytic, possibly expensive.

An alternative is to use 2 identical amplifiers in BTL (bridge tied load) - you drive the 2 amplifiers with opposite phase inputs and hang the speaker between the 2 outputs.

If you Google "BTL amplifier" you should get enough hits to explain it fully.

If you want decent power from a low-ish single rail, there's various off the shelf BTL amp chips - one of my favourites is the TDA7052, there is a suffixed version with DC volume control.

Will it also consume more power during fairly quiet music passages than a split supply?

Oh ok! Is this how it's commonly done with car audio?

Thanks!

Michael

So, even a +6V - 0 - (-6V) supply should be better than +12V-0, right?

Thanks,

Michael

With a bipolar supply (+&- to COM), you get twice the swing and no need for a DC blocking cap which allows you to output down to

0 HZ. (DC)

Jamie

In a case like that, you would get the same results on the swing level if you were to us a capacitor decoupled type over a DC coupled type.

However, using no capacitor is still better because it removes a component from the circuit that could hamper the flat response output, in other words, the capacitor is not going to be the same Z (impedance) across the frequency spectrum.

Jamie

** The electro cap in series with the speaker charges up to half supply voltage at switch on - removing DC from the speaker and then holds that voltage during operation. Current can flow into the speaker from the amplifier in both directions, just as it would with a split supply design. ** Nope.

Single rail amps have a big advantage when the supply voltage has to be regulated or is derived from a battery. Also, it eliminates the possibility of DC flowing into the speaker because of a fault in the amp.

.... Phil

Ok! Thanks for the clarification!

Michael

Ooh. Ok!

Note that with a bridge configuration you are effectively doubling the voltage swing, which quadruples the available power output. This is especially useful in systems (like autos) with low voltage supplies.

Best regards,

Bob Masta DAQARTA v7.50 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, Pitch Track, Pitch-to-MIDI FREE Signal Generator, DaqMusiq generator Science with your sound card!

As for quiescent current - it depends how well the 2 amplifiers are matched/biassed, one or two of the 7052 chips I've used weren't all that good.

Bridge amplifiers are pretty common in cars, but the real in car audio fanatics buy bloody great inverters that produce split rails of up to 56V each - and kill the ignition at high volume if the alternator isn't running at max output!

Philips made a chip that also drives a charge pump voltage booster from the regular output, internal circuitry detects the onset of clipping and activates an extra pair of transistors that run off the capacitors charged by the charge pump - no doubt other manufacturers have copied the idea by now.

On one occasion I found a TDA7052 BTL amp running from the +5V rail on an Olivetti laptop board.

He makes a good point about a fault causing DC current through the speaker - any decent split rail amp will have a protection circuit and relays to isolate the load if it detects a fault.

It would be a piss poor design if a bridge or DC output amp didn't initiate silently.

Jamie